1. Field of the Invention
The present invention relates to an optical low pass filter having an infrared shield film (hereafter referred to as a “shield film”), and more particularly to a shield film which blocks infrared light and suppresses ripple (variations in transmittance) in an opaque zone.
An optical low pass filter is incorporated, for example, in front of a CCD in a camera device to cancel color Moire fringes and the like. Optical low pass filters generally block infrared light and pass only light of short wavelengths to prevent negative effects on imaged color (read color) in the optical system. The shield film which blocks the infrared light is normally formed from laminations of optical thin films of materials of low and high refractivity.
2. Description of the Related Art
FIG. 3A and FIG. 3B are partial enlarged cross-sectional views illustrating an example of a conventional optical low pass filter.
As shown in FIG. 3A and FIG. 3B, the optical low pass filter is provided with a shield film 2 which blocks infrared light on, for example, one primary surface of an optical plate 1 comprised of a single layer or laminates of quartz or glass and the like. The optical plate 1 fundamentally transmits light of all infrared wavelengths λ (wavelength λ=650 nm or greater). The shield film 2 provides an opaque zone by absorbing light of wavelength λ between 650 nm and 1050 nm being the infrared region. As a result, spectral characteristics as shown in FIG. 4 are obtained, wherein wavelengths λ between approximately 400 nm and 650 nm being visible light are a transparent zone.
Normally, as shown in FIG. 3B, the shield film 2 comprising a first laminated film 4a and a second laminated film 4b provided in sequence is formed on at least one primary surface of the optical plate 1. The first and second laminated films 4a and 4b are both formed by alternately laminating transparent optical thin films 3 of high refractivity material (3H) and low refractivity material (3L) by, for example, vapor deposition in, for example, twenty of more layers. Here, the low refractivity material comprises for example, silicon oxide (SiO2), and the high refractivity material comprises titanium oxide (TiO2).
Here, the optical film thicknesses of the optical thin films 3 of the first and second laminated films 4a and 4b are “a” and “b”. Generally, the optical film thickness “a” of the optical thin film 3 in the first laminated film 4a is between 1.0 and 1.3, and the optical film thickness “b” of the second laminated film 4b is between 1.4 and 1.7. Here, the optical film thickness is expressed as the product of the refractivity n and the physical thickness d (n×d) of the high refractivity material or the low refractivity material, and the optical film thickness λ/4 at the design wavelength λ is normally shown as 1.0.
Furthermore, the design wavelength λ is 650 nm being the infrared region, and the optical film thicknesses “a” and “b” are set with λ/4 (162.5 nm) as “1”. The opaque zone for the first laminated film 4a is for example between 650 nm and 870 nm, and the opaque zone for the second laminated film 4b is for example between 870 nm and 1050 nm. As a result, spectral characteristics which block infrared light in the aforementioned opaque zone between 650 nm and 1050 nm can be obtained (see “Skillful Use of Design and Simulation Software—Revised Edition”, pp 150–152, published by Optronics Ltd., May 14, 2003).
However, with the optical low pass filter of the conventional example of the above described construction, there is a problem in that, as shown in FIG. 4, infrared light is not sufficiently blocked in the wavelength λ 870 nm region at the interface between the first laminated film 4a and the second laminated film 4b, and ripple (variation in transmittance) occurs. Moreover, this ripple has a negative effect on imaged color in the optical system, as described above.
An object of the present invention is to provide an optical low pass filter which has spectral characteristics where ripple in the opaque zone which blocks infrared light is suppressed.